Page 461 - Elements of Chemical Reaction Engineering Ebook

432 Steady-State Nonisothermal Reactor Design Chap. 8

the species that is the basis of calculation [i.e., species A Cjoules per mole of A
reacted)].
Substituting Equation (8-14) into (8-13) and reverting to summation
notation for the species, Equation (8-13) becomes

Combining Equations (8-10) and (8-IS), we can now write the steady-state
[i.e., (dksYs/dt = 0)] energy balance in a more usable form:
One can use this
form of the steady-
state energy
balance if the
enthalpies are
available
If a phase change takes place during the course of a reaction, this form of the
energy balance [i.e., Equation (8-16)] should be used, e.g., Problem 8-3.

8.2.4 Dissecting the Enthalpies

We are neglecting any enthalpy changes on mixing so that the partial molal
enthalpies are equal to the molal enthalpies of the pure components. The molal
enthalpy of species i at a particular temperature and pressure, H,, is usually
expressed in terms of an enthalpy of formation of species i at some reference
temperature TR, H,"(T,), plus the change in enthalpy that results when the tem-
perature is raised from the reference temperature to some temperature T, AHQ, :

The reference temperature at which Ho is given is usually 25°C. For any sub-
stance i that is being heated from T, to T2 in the absence of phase change,

Typical units of the heat capacity, Cpi, are
Btu caI
or
) = (mol of i ) (K) (lb mol of i ) (R) Or (mol of i ) (K)

Example 8-2 Relating HA(T) Hi (TR)
to
Species A is a solid at 25°C. Its enthalpy of formation is Hi (298 K). Write an
expression for the enthalpy of substance A in the gaseous state at temperature T.

I Solution
the
enthalpy when Here, in addition to the increase in enthalpy of the solid, liquid, and gas from the
phase changes are temperature increase, one must include the heat of melting at the melting point,
involved AHrnA (T,), and the heat of vaporization at the boiling point, AHVA(Tb).